Autostereoscopic display

ABSTRACT

An autostereoscopic display device comprises a display array with addressable pixels. The display device comprises a means for providing collimated light ( 60 ) emitted by the pixels of the display array ( 61 ), and a cylindrical lens ( 63 ) for focusing the image displayed on the display array on a display screen ( 66 ). A scanning means ( 64 ) is provided to sequentially scan over said display screen, and means ( 69 ) for changing the image information on the display array ( 61 ) in a rate corresponding to the frequency of scanning of the openings in the display screen are also provided.

The invention relates to an autostereoscopic display device comprising adisplay array comprising a number of addressable pixels, and a means foraddressing the pixels in the display array.

BACKGROUND OF THE INVENTION

An autostereoscopic display device of the type described in the openingparagraph is known from U.S. Pat. No. 5,969,850.

Basically, a three dimensional impression can be created by using stereopairs (two different images directed at the two eyes of the viewer),holographic techniques, or multiple planes in the displays. With themultiplanar techniques, a volumetric image is constructed, in which the2D pixels are replaced by so-called voxels in a 3d volume. Adisadvantage of most multiplanar displays is that the voxels producelight, but do not block it. This leads to transparent objects, givingquite literally a ghostly and unpleasant appearance to the displayedimages.

Stereoscopic displays do not suffer from this problem. There are severalways to produce stereo images. The images may be time multiplexed on a2D display, but this requires that the viewers wear glasses with e.g.LCD shutters. When the stereo images are displayed at the same time, theimages can be directed to the appropriate eye by using a head mounteddisplay, or by using polarized glasses (the images are then producedwith orthogonally polarized light). The glasses worn by the observereffectively route the views to each eye. Shutters or polarizer's in theglasses are synchronized to the frame rate to control the routing. Toprevent flicker, the frame rate must be doubled or the resolution halvedwith respect to the two dimensional equivalent image. A disadvantagewith such as system is that the two images produce only a limited “lookaround” capability. Furthermore, glasses have to be worn to produce anyeffect. This is unpleasant for those observers who are not familiar withwearing glasses and a potential problem for those already wearingglasses, since the extra pair of glasses do not always fit.

Instead of near the viewers eyes, the two stereo images can also besplit at the display screen by means of splitting screen such as aparallax barrier, as e.g. shown in U.S. Pat. No. 5,969,850.

Although these displays are autostereoscopic in the sense that nospecial glasses are required to view the 3D image, they often work onlyfor one viewer at a fixed position in space. The viewing zone is verynarrow. Outside the viewing zone, the observer sees multiple images or astereo inversion, leading to a very unpleasant view. In practice thismeans that for many application, for instance in living rooms, theviewing zone is so small that the viewer has to be seated at oneparticular spot to be able to see a 3D image. Solution which offermulti-view images do so at the cost of resolution.

The device known from United States Patent U.S. Pat. No. 5,969,850offers a solution to the narrow viewing zone problem by using a dynamicparallax barrier, i.e. a parallax barrier wherein the barrier slits moveacross the screen.

Although it is possible to obtain a multiview autostereoscopic displayin the manner described in U.S. Pat. No. 5,969,850, a drawback of theabove described principle is a lack of efficiency. Only a small amountof the light emitted passes the dynamic parallax barrier. For instance,if we use a parallax barrier display with 1000 slots (so, 1000subframes), we have a horizontal resolution of 1000 pixels. However, thelight shines on the whole backside of the parallax barrier and thelatter blocks 99:9% of the light. So, for a television application weneed at least a very high light intensity to get a sufficiently brightpicture. Although the efficiency can be improved by making several slotsof the dynamic barrier transparent at the same time, the principleproblem of a very poor efficiency remains and in order to be able to useseveral slots the amount of different viewing directions has to becompromised

SUMMARY OF THE INVETION

It is therefore an object of the invention to provide an alternative tothe known device enabling autostereoscopic views and an improvedefficiency.

To this end the device in accordance with the invention is characterizedin that the display device comprises a means for providing collimatedlight emitted by the pixels of the display array, a cylindrical lens forfocusing the image displayed on the display array in a directionperpendicular to the longitudinal axis of the cylindrical lens, thedevice further comprising a display screen comprising a number ofopenings upon which the image displayed on the display array is inoperation focused, and a scanning means to sequentially scan over saidopenings on the display screen, and means for changing the imageinformation on the display array in a rate corresponding to thefrequency of scanning of the openings in the display screen.

Collimated light means, within the concept of the invention, light whichis confined to within a relatively narrow angle, typically less than 10degrees, preferably less than 5 degrees and most preferably withinapproximately 2 degrees. Within the framework of the invention“collimation” means collimation in at least one direction, the directionof scanning, not necessarily in two direction, i.e. not necessarily alsoin a direction perpendicular to the scanning direction. In practice thiswill often mean collimation in the horizontal direction (left-right),whereas collimation in a vertical direction (up-down) will or can bemuch less or not apparent.

The viewing directions of every vertical line on the display screen areconstructed by projecting a two dimensional display array on onevertical line formed by a cylindrical lens by means of a positivecylindrical lens in between the display array and the display screen.During a frame time, the openings on the display screen are scanned fromleft to right (or the opposite) by means of scanning means, such as e.g.and preferably a rotating mirror or polygon. For every vertical line anew picture is displayed on the two-dimensional display screen. So, thedisplay array has to be sufficiently fast like, i.e. when e.g. a framerate of 50 Hz is used and 1000 cylindrical lenses are used on thedisplay screen the rate of change of the display array has to be50*1000=50 kHz. Such devices exits e.g. LCD's based on ferro-electricliquid crystals and micro-mirror arrays.

The device in accordance with the invention has the advantage that ahigh efficiency is obtained since all the light emitted by the displayarray is used, furthermore the device can be used in 3D mode, as well asin a 2D mode, by simply changing the information sent to the displayarray, thus it is 2D-3D compatible. The device allows a large number ofviewing zones, without having to use goggles, or having a very poorefficiency.

To increase the total viewing angle, the use of cylindrical lenses on ornear the display screen is preferred. Without these elements the viewingangle would be too small for a domestic television application.

In addition, the use of a lens in between the display array and thedisplay screen to reduce the optical path length is preferred to reducethe depth of the display device.

To get a good display screen performance, a good synchronization of the(sub)frame rate of the two-dimensional display and the scanning by thescanning means e.g. the speed of rotation of a rotating mirror orpolygon is preferred. In the preferred class of embodiments this isachieved by means of a feed-back mechanism that uses an index signal ofindex light sensors (e.g. photo-diode(s)) to detect the position of thelight beam on the back side of the display screen.

In a preferred class of embodiments the device is provided with a slitshadow mask in between the scanning means and the display screen. To geta good performance its must be ensured that the image intended to fallon a cylindrical lens, indeed falls on the cylindrical lens. By using aslit shadow mask in between the scanning means and the display screenslight misalignments of the light path are counteracted, be it at a costof reducing the brightness of the display.

In another preferred class of embodiments the display screen is providedwith index light sensors and the side facing the scanning means, and thesignal from the index light sensors is fed back to the scanning means.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 illustrates the basic principle of a display device.

FIG. 2 illustrates the basic principle of a parallax barrier splittingtwo stereo images.

FIGS. 3A and 3B illustrate the principles of a barrier and a lenticularscreen.

FIG. 4 illustrates the problem encountered with a basic parallax barrierdisplay.

FIG. 5 illustrates a known multi-view devices

FIG. 6 illustrates a display device in accordance with the invention

FIGS. 7, 8 and 9 illustrate further examples of display devices inaccordance with an embodiment of the invention

The figures are not drawn to scale. Generally, identical components aredenoted by the same reference numerals in the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a display device. For a three-dimensional televisionone needs a large number of viewing directions in which light is emittedindependently. In a domestic application, the viewing distance isapproximately 3 m and people sits on a couch that is approximately 3 mwide, see illustration 1. So, a viewing angle of at least 60-degrees isneeded. Our eyes are positioned at 6:5 cm from each other. To arrive atdifferent pictures for each eye, the display needs to emit light in atleast 3 m/6:5 cm=300/6:5=50 directions. To arrive at a three-dimensionalpicture without discontinuous transitions if one moves one's head, athree-dimensional television should emit light in much more than 50, sayat least 100, directions.

FIG. 2 illustrates the basic principle of a parallax barrier splittingtwo stereo images. The vertical lines of two stereo images arealternatingly displayed on, e.g., a spatial light modulator (e.g. a LCD)with a back light. The grating structure of the parallax barrier 7ensures that each eye of the viewer 4 sees the appropriate stereo image(5, 6).

FIG. 3A illustrate the use of a parallax barrier.

In a conventional barrier auto-stereoscopic display system, a barrier 31is disposed in front of a display array 32. The left and right images ofa stereo pair of images are sliced into vertical strips. The strips 32Lof the left image and the strips 32R of the right image are alternatelydisposed on array 32. Slots 31A are formed in barrier 31. Slots 31A arepositioned so that the left eye 4L of an observer can see only strips32L of the left image and the right eye 4R can see only strips 32R ofthe right image of the pair. The observer reconstructs the full image inthree dimensions.

Referring now to FIG. 3B, a similar principle is explained in whichbarrier 31 is replaced by a lenticular lens screen 33 having an array ofvertical cylindrical lenses 33A each corresponding to a different pairof left and right image strips 32L and 32R. In operation each lensdirects the left eye 4L of an observer onto a left image strip 32L andthe right eye 4R of the observer onto a right image strip 32R.

FIG. 4 illustrates the problem of a basic stereoscopic device. A viewerwhich is not seated within the right viewing zone is confused. Theviewing zone is very narrow. Outside the viewing zone, the observer seesmultiple images or a stereo inversion, leading to a very unpleasantview. In practice this means that for many application, for instance inliving rooms, the viewing zone is so small that the viewer has to beseated at one particular spot only to be able to see anything. Forliving room use this is far from optimal since only one viewer can seethe 3D image and then only when seated on one spot.

FIG. 5 illustrates schematically a device as known from United StatesPatent U.S. Pat. No. 5,969,850. In this device a solution to the narrowviewing zone problem is offered by using a dynamic parallax barrier 51,i.e. a parallax barrier wherein the barrier slits move across thescreen.

Although it is possible to obtain a multiview autostereoscopic displayin the manner described in U.S. Pat. No. 5,969,850, a drawback of theabove described principle is a lack of efficiency. Only a small amountof the light emitted passes the dynamic parallax barrier. For instance,if we use a parallax barrier display with 1000 slots (so, 1000subframes), we have a horizontal resolution of 1000 pixels. However, thelight shines on the whole backside of the parallax barrier and thelatter blocks 99:9% of the light. So, for a television application weneed at least a very high light intensity to get a sufficiently brightpicture. Although the efficiency can be improved by making several slotsof the dynamic barrier transparent at the same time, the principleproblem of a very poor efficiency remains and in order to be able to useseveral slots the amount of different viewing directions has to becompromised

Besides the deterioration of resolution which is present for lenticularlenses and parallax barriers alike, the light transmission through aparallax barrier is also greatly reduced, since only one of about 100vertical lines of the barrier is transparent. This blocking of (morethan) 99% of the light results in an extremely inefficient display.

Thus the problem remains that an good 3D display should beautostereoscopic in the sense that no glasses are required yet have agood light output.

Furthermore it preferably has a “look-around” capability to avoidproblems with focussing of the eye and headaches. Preferably thiscapability should be intrinsic to the display, without additional meansfor tracking the head of the viewer. For TV applications, the displaymust also have a multi-viewer capability. Finally, the 3D display shouldalso be 2D compatible. The above autostereoscopic displays withmultiviewer capability can in principle be made by means of a lenticularscreen or a parallax barrier, but at the cost of a greatly reducedresolution.

To this end the device in accordance with the invention is characterizedin that the display device comprises a means for providing collimatedlight emitted by the pixels of the display array, a cylindrical lens forfocusing the image displayed on the display array in a directionperpendicular to the longitudinal axis of the cylindrical lens, thedevice further comprising a display screen comprising a number openings,preferably provided with cylindrical lenses upon which the imagedisplayed on the display array is in operation focused, and a scanningmeans to sequentially scan over openings (preferably over saidcylindrical lenses) on the display screen, and means for changing theimage information on the display array in a rate corresponding to thefrequency of scanning of the openings in the display screen.

The principle of the device in accordance with the invention isschematically shown in FIG. 6, which shows a simple embodiment of theinvention.

The viewing directions of every vertical line in the display areconstructed by projecting collimated light from light source 60 passingthrough a two dimensional display array 61 on one vertical line 62 bymeans of a positive cylindrical lens 63. During a frame time, thecolumns are scanned from left to right (or the opposite) by means of arotating mirror or polygon 64. For each vertical line 62 a cylindricallens 65 is provided on a display screen 66. For every vertical line 62on the screen 66 a new picture is displayed on the two-dimensionaldisplay 61. The cylindrical lenses 65 on which this line (which inreality comprises a full picture) falls expands in preferred embodimentsinto light rays 67. The display array has to be sufficiently fast tochange the picture on the display array in tune with the scanning overthe openings (or in preferred embodiments the cylindrical lenses). LCD'sbased on ferro-electric liquid crystals are e.g. sufficiently fast.

To increase the total viewing angle, the use of optical elements likecylindrical lenses 65 at the screen position is preferred. Without theseelements, i.e. when the screen is simply provided with openings theviewing angle would be too small for a domestic television application.In addition, additional optical elements such as a lens 68 are preferredto reduce the depth of the display.

The display device in accordance with the invention does not have theefficiency problems as discussed for the display of FIG. 5. In thedisplay device in accordance with the invention all (or at least almostall) light from the display array 61 is used to construct a picture.However, this light has to be collimated. A suitable collimated lightsource might be an LCD-projector (“beamer”) or laser light might beused. In the latter case, one might using a scanning laser beam togenerate the two-dimensional picture. The display device in accordancewith the invention is backward compatible. Normal video can be shown byhaving the same luminance information in every directional view. I.e.every pixel in a row of the two-dimensional display array shows the sameinformation. It is within the concept of the invention also possible touse a display array emitting light into all directions and themcollimating said light, i.e. putting a light collimating element inbetween an emitting light array and the rotating mirror or polygon.However, this usually means that only a part of the emitted light iseffectively used, often only using a few to a few tens of the emittedlight. However, even in such embodiments the efficiency of the displaydevice in accordance with the invention far exceeds the efficiency ofthe known devices. The array is coupled to a control device 69 to makean image on the display array.

To get a good front of screen performance, it is preferred tosynchronize the (sub)frame rate of the two-dimensional display and therotating angle of the mirror or polygon. For instance, this can beachieved by means of a feed-back mechanism that uses a photo-diode(s) todetect the position of the beam on the screen.

FIG. 7 illustrates such an embodiment. In this fig. a part of screen 66is enlarged to show a photo-sensitive element, such as a photo-diode 71on the screen. The signals from the photo-sensitive element 71 are sendto a control device 72 which via a feed-back mechanism regulates therotating mirror or polygon 64. It is remarked that “rotating” within theconcept of the invention comprises any movement which scans the image ofthe display array over the display screen. In the simplest embodimentsthis is a rotating movement, it may, however also be a tilting orrocking (back-and forth) movement.

In another preferred class of embodiments the device is provided with aslit shadow mask in between the scanning means 64 and the display screen66. To get a good performance its must be ensured that the imageintended to fall on an opening (such as e.g. on cylindrical lens 65),indeed falls on the cylindrical lens. By using a slit shadow mask inbetween the scanning means and the display screen slight misalignmentsof the light path are counteracted, be it at a cost of reducing thebrightness of the display. FIG. 8 schematically illustrates a detail ofa device in accordance with such an embodiment. In front of the lenses65 (i.e. in the light path between the scanning means (such as e.g. arotating mirror) and the lenses a slit shadow mask 81 is positioned. Theshadow mask ensures that the light falls only on the lenses. Thisincreases the performance of the device, but at the cost of lightoutput.

It will be clear that within the concept of the invention manyvariations are possible.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. The invention resides in each and every novelcharacteristic feature and each and every combination of characteristicfeatures. Reference numerals in the claims do not limit their protectivescope. Use of the verb “to comprise” and its conjugations does notexclude the presence of elements other than those stated in the claims.Use of the article “a” or “an” preceding an element does not exclude thepresence of a plurality of such elements.

For instance, whereas in the so far shown exemplary embodiments only onedisplay array is used, in embodiments of the display device inaccordance with the invention more than one display array could bescanned over the screen or over part of the screen. Such an embodimentis schematically shown in FIG. 9 in which two display arrays 91, 92 arepresent This can be advantageously used to increase the light output orto reduce the refresh rate of the displays.

A further example of a variation on the shown exemplary embodiments isformed by e.g. embodiments in which as a lens (either for lens 63 or 68)a curved mirror surface is used. As is well known a concave mirrorsurface, if properly shaped, acts as a lens, so within the concept ofthe invention “lens” may be a mirror surface acting as a lens. Whenusing a lens the lens may be to some extent dynamic, e.g. by changingthe position of the lens (back and forth towards the screen for lens 68or towards the rotating mirror for lens 63) depending on the line on thescreen to which the light is directed. A better focusing is therebyobtainable. When using a mirror surface the position as well as thecurvature may be dynamic.

In short the invention may be described by:

An autostereoscopic display device comprises a display array withaddressable pixels. The display device comprises a means for providingcollimated light (60) emitted by the pixels of the display array (61),and a cylindrical lens (63) for focusing the image displayed on thedisplay array on a display screen (66). A scanning means (64) isprovided to sequentially scan over said display screen, and means (69)for changing the image information on the display array (61) in a ratecorresponding to the frequency of scanning of the openings in thedisplay screen are also provided.

1. An autostereoscopic display device comprising a display array comprising a number of addressable pixels, and a means for addressing the pixels in the display array, characterized in that the display device comprises a means for providing collimated light (60) emitted by the pixels of the display array (61), a cylindrical lens (63) for focusing the image displayed on the display array in a direction perpendicular to the longitudinal axis of the cylindrical lens, the device further comprising a display screen (66) comprising a number of openings upon which the image displayed on the display array is in operation focused, and a scanning means (64) to sequentially scan over said openings on the display screen, and means (69) for changing the image information on the display array (61) in a rate corresponding to the frequency of scanning of the openings in the display screen.
 2. An autostereoscopic display device as claimed in claim 1, characterized in that the device comprises in, near to or on the display screen (66) cylindrical lenses (65).
 3. An autostereoscopic display device as claimed in claim 1, characterized in that the display device comprises in between the scanning means and the display screen a lens (68).
 4. An autostereoscopic display device as claimed in claim 1, characterized in that the display device comprises index light sensors (71) on or near the display screen.
 5. An autostereoscopic display device as claimed in claim 1, characterized in that the display device comprises a shadow mask (81) in between the scanning means and the display screen. 